Electrical borehole heater



May 5, 1954 R. E. KU|-|r-;TK.1=\ ETAL ELECTRICAL BoREHoLE HEATER 4 Sheets-Sheet 1 Filed Deo. 5o, 1959 May 5, 1964 R. E. KUNETKA ETAL ELECTRICAL BoREHoLE HEATER 4 Sheets-Sheet 2 Filed Dec. 30, 1959 N.. .wwwsmwww s QQXWT 1%/ JZ/,d

MF/Tl n@ May 5, 1964 Filed Dec. 30, 1959 R. E. KUNETKA ETAL ELECTRICAL BOREHOLE HEATER 4 SheP'cS-Sheekl 3 May 5, 1964 R. E. KUNETKA ETAL 3,131,763

ELECTRICAL BOREHOLE HEATER 4 Sheets-Sheet 4 Filed Dec. '50, 1959 United States Patent O 3,131,763 ELECTRICAL BBREHLE HEATER Robert E. Kunetha, Houston, Valery N. Bednarsiri, Beilaire, and Billy H. Terrell and Charles D. Woodward,

Houston, Tex., assigner-s to rlexaco lne., New York,

FLY., a corporation of eiaware Filed Dec. 3i), i959, Ser. N 862,876 6 Claims. (Ci. l @-66) This invention relates to the treatment of underground formations and, more particularly, this invention relates to a heater suitable for use in boreholes.

Various techniques have been proposed for the recovery of petroleum from underground formations. One of the techniques involves in situ combustion. In this technique, as practiced in secondary recovery operations, the .petroleum producing .formation in the vicinity of a borehole or injection well is heated to a high temperature, for example 1000c F., and oxidizing gas, such as air, is supplied to the underground formation. The combustion gases produced around the borehole migrate through the formation to an output well or wells leading from n e formation from which a petroleum product is removed.

In accordance with this invention `an improved electrical heater is provided for initiating combustion of the hydrocarbon material in the subsurface formation, which is of a simple, rugged construction capable of withstanding greater thermal shock `due to rapid temperature changes than can prior art electrical borehole heaters, and can operate at higher temperatures for longer periods of time. Heat from the heater is introduced into the borehole at a relatively rapid rate until there exists a high temperature zone around the borehole suiiicient in extent 4to sustain the in situ combustion process.

The heater, in addition to being useful in secondary recovery operations, may also be used for the carbonization of the area around the well as a means of making it preferentially wettable `to oil and thereby to improve permeability. Furthermore, the Iheater may lbe used for the removal of moisture in subsurface formations in the vicinity of the well bore and for any other application where a concentrated high temperature is desired. The heater of the present invention comprises an electrically resistive wire wound in the form of a helix threaded through annular insulators carried by an elongated supporting structure.

`For a better understanding of the invention, reference may be had -to the accompanying drawing in which:

PIG. l is a vertical sectional view through a portion of a borehole traversing subsurface formations showing therein the general arrangement of the apparatus used in accordance with this invention,

FlGS. 2, 2a, 2b, 2c and 2d illustrate in more detail the apparatus shown in FIG. l,

FIG. 3 illustrates the electrical circuitry used in the apparatus of this invention,

FiG. 4 illustrates the heater element supporting frame of the apparatus of the invention,

FG. 5 is a cross-sectional View of the apparatus at 5 5 of FIG. 2b,

FIG. 6 is a cross-sectional view of the apparatus at 6 5 of FG. 2b,

FG. 7 is a cross-sectional lview of the apparatus taken at 7-7 of FIG. 2b, and

FIG. 8 is a cross sectional view of the yapparatus taken at y8 8 of FiG. 2c.

Referring to the drawing wherein like reference numerals refer to similar elements illustrated in the various figures, there is shown in FIG. l a borehole 10 traversing a producing formation l2. The upper portion of the bore- 3,131,763 Patented May 5, 1964 ice hole l0 is lined with a casing lli having La closed casing or braden head i6 yat the upper end thereof.

The well heating system or electrical borehole heater of the present invention which is suitable for use to heat a subsurface formation, such as formation l2, in the Vicinity of a borehole comprises `a heater housing 18 which includes a thin sheath `2% having a 41/2" outside diameter and a suitable length depending upon reservoir thickness, for example, l5 feet, a bottom plug 22 attached to the sheath 20 so as to form a seal therewith and a heater head 24 attached to the upper end of the sheath 2Q, a power cable 26 which is connected to a pair of heater elements 149 and 142 disposed within the sheath 20, temperature signal cable 28 which contains seven copper conductors for transmitting electrical signals indicative of temperature variations from four thermocouples 29, Si), 3l and 33 and a thermistor v.76 to the earths surface through suitable openings in the casing head 16, and a protective cable tubing 32 attached at its lower end to the heater head 24 and at its upper end to a cable cross-over 34 having an intake structure for receiving `the cables 26 and 28 so as to introduce these cables into the protective tubing 32. The cable cross-over 34, protective cable tubing 32, and the heater housing l are supported Iin the borehole by well tubing 35 having a pup-joint 33 at the lower end thereof, which tubing is suspended from the casing head .16. The power cable 25 and the signal cable 2S are supported in the well by a plurality of ties 2.1 made of suitable banding material attached to the well tubing 36 at longitudinally `spaced points. Connected to the upper end `of the Well tubing 36, preferably through a valve 39, is a gauge 4u yfor indicating the pressure in the borehole 1%. A pipe 4-2 having a valve 44 is connected to the upper portion of the casing 1d to introduce an oxidizing gas, for example, air into the borehole. The heater housing l has a heating section 27 located in the lower portion thereof and a heat baille section 23 located in the upper portion thereof. The four therrnocouples 29, 3d, 3l and 33` are disposed in the heater housing 13 Iat longitudinally spaced apart points.

As shown in more detail in FlGS. 2, 2a, 2b, 2c, and 2d, the apparatus or" the present invention includes a hanger head do having a shoulder 47 at the upper portion thereof as shown herein supported on the edge of a lirst pipe 49 disposed between the cable cross-over 34 and the pup-joint 3S of the well tubing 35. A top plug 43 having a circumferential groove therein containing an O-ring 45 is disposed within the lower end of the pupjoint 38 to provide a seal between the interior of the pup-joint 3S and the interior of the rst pipe 49. The pup-joint 33 and the first pipe @-9 are coupled together by a lirst coupler 35. A chain hanger 43 having a loop 51 at the lower end is inserted through an opening or passageway in the hanger head d6 and is supported by a wing nut Sil threaded thereon. The power cable 26, which includes two relatively high potential conductors 17 and 19, is introduced into the cable cross-over 34 through a passage S2 in a cable cross-over head 54. A packing 56 and a packing screw gland 58 surround the cable 26 witln'n the cable cross-over head 54 so as to provide a fluid-tight joint. A set screw 6i) is inserted into the cable cross-over head 54 to restrain the power cable 26 in the passage 52. The signal cable 2S is similarly introduced into the cable cross-over 34 through a passage 62, a packing 64 and packing screw gland 66. The crossover head 54 also has a gas entry port and valve 25 communicating with the passage in which the chain hanger 48 is disposed to introduce an inert gas into the cable cross-over 34 and the protectice cable tubing 32. The valve 2S may be protected by a suitable pipe Y 3 plug 59; An O-ring 68 is disposed in a circumferential groove in the cross-over head 54 so as to form a fluidtight seal between the cross-over head 54 and a cylindrical sheath 7G of the cable cross-over 34. Attached to the loop S1 of the chain hanger 4S is a chair; 72 for supporting the cables 26 arid 2S within the cable protective tubing 32. Suitable ties 74, which are made preferably of stainless steel annealed wire, are used to lash the cables 26 and 28 to the chain 72. A thermistor 76 connected to two of the seven copper conductors of the signal cable 2S is mounted within the cable cross-over dV so as to provide an indication of the temperature therein. A cable cross-over adapter 77 having a circumferential groove containing an O-ring 'i9 couples the cable cross-over 34 to the protective cable tubing 32. Thermocouple wires 73 of the four thermocouples 29, 3i?, 31 and 33, which are preferably made of material known by the trade name Alumel and Chromel, are connected to the remaining five conductors of the signal cable 23 in the cable cross-over 34 at a cold junction terminal 75 in the vicinity of the thermistor 76. The power cable 26 and the thermocouple wires 73 are supported Within the heater head 24% by means of cable clamps '78 i and a cable bracket iii? attached to a vertical support member S1 mounted on a supporting block 84. Disposed within the heater head 24 is the uppermost or first thermocouple 29, as shown in FIG. 2b. An O-ring 33 is contained in a groove disposed around the lower portion of the heater head 24 to provide a seal between the heater head 24 and the supporting block S4. The supporting block 84 has a pair of O-rings 92 to form a seal between the supporting block S4 and the sheath 29 of the heater housing 18. The supporting block 84 also has a gas entry port and valve 86 communicating with a passage 8S which is used to introduce an inert gas into the heater housing 1S. The valve S6 may be protected by a suitable pipe plug Qil. The supporting block 34 further includes a shoulder 85 supported by the upper edge of the sheath 20. The supporting block S4- is firmly held in position within the sheath 2l) by a plurality of set screws 94,k as shown more clearly in FG. 7 of the drawing.

FIG. 2b or" the drawing shows the two power leads 17 and 19 passing through passageways 1116 and 162 in the supporting block S4, a seal being provided between each ofthe power leads 17 and 19 and the supporting block S4 by a packing 1194 and a packing screw gland 166. The wires 73 of the three thermocouples 3G, 31 and 33 disposed in the heating section 27 of the heater are fed through a passageway 168 in the supporting block 84 and a packing 110 and a packing screw gland 112 provide a seal between the thermocouple wires 73 and the supporting block 84.

A heater element support frame 114 is suspended from the supporting block 84 by means of a plurality of frame hangersV 116. The frame hangers 116 are preferably welded to the supporting block 84 and the support frame 114 is secured to the frame hangers 116 by means of a nut and bolt arrangement 118, as shown in FIG. 6 as well as in FIG. 2b of the drawing. The supporting frame 114 comprises an elongated plate 120 and two elongated L-snaped members 122 and 124 welded to opposite faces of the plate 126 to provide a frame having a cruciform transverse cross-section, as shown more clearly in FIG. 4 of the drawing. The frame 114 extends from the supporting block 84 longitudinally through the heat baille section 23 and the heating section 27 of the heater housing 18 to a point spaced a given distance from the top of the bottom plug 22 toallow for differential eXpansion associated with temperature changes in the heater Vhousing 18. The outer edges of the cruciform frame V114 are spaced from the sheath 20 of the heater housing 13 also to allow for dierential expansion due to temperature changes. The outer sheath 20 of the heater housing 18 and the portion of the supporting frame 114 within the heating section 27 of the heater housing 18 are preferably made of Ineonel which includes Ni(77.0). Cu(0.2), Fe(7.0), Mn(0.25), Si(0.25), C(0.08), S(0.0G7), and Cr(l5.0). The portion of the supporting frame 114- within the baille section 23 is preferably made of stainless steel. A plurality of lead-in brackets 126 each having a U-shaped edge opening to deine a saddle for receiving an annular insulator 127, preferably made of a ceramic material such as that known by the trade name Aisimag #222, for restraining one of the two power conductors or leads 17, 19 are mounted on the supporting frame 114, as shown in FIGS. 2b, 2c, 4 and 5. Also mounted on the supporting frame 114 are an upper heat baille 123 and a lower heat baille 13? suitably disposed in the heat Vbaille section 23 to protect the heater head 24 and the cables 26 and 2S at the upper portion of the heater from the high temperatures produced in the heating section 27. The heat bailles 128 and 130 are preferably made of insulator-tiberium ceramic liber paper. The thermocouple wires 73 are held in position against the supporting frame 114 but insulated therefrom by means of a plurality of straps 131. The thermocouple wires `V'73 are insulated throughout their entire length to a point spaced a short distance from their hot junction terminals, a iirst hot junction terminal 30 being located at the upper end, a second hot junction terminal 31 at the mid-point and a third hot junction terminal 33 at the lower end of the heating section 27.

In the heating section 27 of 4the heater the power conductors 17 and 19 are terminated at power lead terminals 132 and 134, respectively, to which 'they may be welded. A plurality of saddles vare formed in each of the four outer edges of the supporting frame 114 in the heating section 27 by removing a cup-shaped or U- shaped portion therefrom, the spacing between the saddles-115 being preferably greater at the mid-portion (not shown) than at the ends of the heating section 27. Annular insulators 127 are also inserted in each of the saddles 115 and held therein by retaining lingers or arfcuate arms 138 made by deforming a portion of the frame 114 embracing the annular insulators 127 at their outer periphery. This structure is consistent for all saddles 115 although only specioally illustrated in FIG. 4 and the upper left hand few saddles of FIG. 2c, for sake of convenience.

The heating section 27 contains two heater elements 140 and 142. Each of these heater elements is wound 'tin the form of a helix having a minor constant or uniform radius forming a `tirst coil which coil is in turn then wound in the form of a helix of major constant or uniform radius through the annular insulators 127 in the saddles 115 of the frame 1'14 to -form a second Icoil extending yfrom one of the two power lead terminals 132, 134 located at the upper end of `the heatfing section 27 to the lower end of the frame 114 and returning -to the upper end of the heating section 27 to the other of the two power lead terminals 132, 134, the pitch of each turn of the second coil being such as to receive in complementary fashion the turns of the other second coil. The ends of lthe two heater eletments 140 and 142 are preferably welded to the power lead terminals 132 and 134 to prov-ide electrical connection to the power'leads 17 and 19. The heater elements in the double helical form -are threaded through the annular insulators 127 so as to be supported by land electrically insulated from the supporting frame 114, as shown in FIGS. 2c, 2d, and 8. Each of the heater elements or wires 140 and 142 Vis preferably 175 long, formed in coils 40.5 feet long known by the trade name of #l2 Iellil` Alloy K wire. The heater elements 149 and 142 are connected in parallel and each is grounded at its electrical cen-V ter to the supporting lframe 114. This circuit arrangemeut provides a heater resistance of l1 ohms and has arid made of what is i a rating of approximately 16 kw. at 440 volts at a rating for the wire of l5 watts per square inch surface area.

The electrical circuit of the heater may be more clearly seen in FIG. 3 of the drawing. A suitable power source, which may be, for example, a 480 volt single phase 60 cycle per second source, is connected to a primary winding 156 of a power transformer 151 which has a secondary winding 152 grounded at its mid-point. The secondary winding 152 is connected across a coil 153 which with first and second variable taps 156 and 15S respectively, form an autotransformer 155 for supplying 4an adjustable voltage between the power conductors 117 and 19 connected to the two heater elements 149 and 142.

A temperature indicating device 160 which may be of any conventional type -is connected to the thermistor 76 ylocated in the cable cross-over 34 and also to each of the therrnocouples 29, 313, 31 and 33. A balancing network 162 which may be, yfor example, any suitable known bridge arrangement is selectively coupled through a three-position switch 164 having `a movable arm m and three stationary contacts zz, b and c, to one of the three thermocouples 39, 31 and 33 located in the heating section 27 oi the heater housing 18. The balancing network 162 is also coupled to a potentiometer 166 which has two iixed terminals 163 and v'176 and an adjustable tap 172. A rst reversible motor 174 is electrically controlled by the balancing network 162 and is mechanically coupled to the adjustable tap 172 of the potentiometer 166. A marker (not shown) of a stripchart recorder 167 is fitted on the tap 172 of the potentiometer v166 so as to produce a graph 176 on a stripchart 178 of the recorder 167 to provide a record of the borehole temperature thereon. The strip-chart 17S is driven at a uniform speed `by the ychart drive motor 182.

A movable Contact 184 is mounted on the tap 172 of potentiometer 166 so as to be electrically insulated therefrom. First and second electrical contacts 186 and 18S lare disposed at fixed spaced apart points so as to be electrically contacted by the movable contact 184 at various time intervals. The lirst electrical contact 186 is connected directly to one terminal of a first 110 volts, 60 cycle per second source 191 and the second electrical contact 13S is connected to the other terminal of the first ll() volt source 191 through a protective resistor 15). A relay 192 has a coil 194 connected between the second electrical contact 188 land the movable contact 184. The relay 192 also has a nst fixed contact 1136 and a cooperating first movable arm 198 normally in an open position yand second and third xed spaced apart contacts 2G13 and 262 and 'a cooperating second movable arm 294 normally contacting the third med Contact 292. The rst movable arm 198 is electrically connected to the movable contact 154 and the iirst ixed contact 196 is yconnected to the first electrical Contact 186. A second ll() volt, 6C cycle per second source 193 has a iirst terminal connected to a first terminal 295 of a second lreversible motor 296. A second terminal of the second 110 volt source 193 is connected through the second movable arm 294 and the lthird iixed .Contact 292 of the relay 152 and through a `first normally closed motor switch 268 to a second terminal 299 of the second reversible motor 2116. A third terminal 21) of the second reversible motor 2116 is connected through a second normally closed motor switch 212 to the Second lfixed contact 261B of the relay 122. A motor switch actuating arm 214 is mounted on the iirst variable tap 156 of the autotransformer 155 to open the fiest and second motor switches 26S and 212 when the desired voltage limits between the variable taps 156 and 158 are reached. The variable taps 156 and 15S of the autotransformer 155 are mechanically coupled to the second reversible motor 2156. The rst motor switch 208 .is disposed -in cooperation with the actuating arm 214 so as to provide a 'lower voltage limit for the heater elements 146 and 142 and the second motor switch 212 is disposed in cooperation with the actuating arm 214 so as to provide an upper voltage limit for the heater elements and 142.

In operation the electrical heater elements 140 and 142 may be assembled within the heater housing 18 at any convenient location but preferably without connecting thereto the power and signal cables 26 and 25 which may be done at the well site. The temperature indicating device 160, the strip-chart recorder 167, the balancing network 162, the relay 192 and the first and second reversible motors 174 and 296 may be installed in a vantype truck for ease of operation and protection from the elements. At the well site the necessary length of the insulation coated power cable 26, for example, a Teflon cable, is connected to the power leads 17 and 19 in the heater head 24. The heater head 24 is then attached to the heater housing sheath 20 and the interior of the heater housing 1S is pressurized through valve 86 located in the support block S4 to approximately 200 p.s.i.g. to protect the housing 18 from high borehole pressure. The power and signal cables 26 and 28 and the chain 72 are taped or tied together and threaded through successive joints of the protective tubing 32 for approximately 200 feet and, preferably, the number of feet necessary to prevent the cable cross-over 34 from being immersed in the borehole liquid. The heater housing 1S and its contents are then placed in the borehole and the protective tubing 32, with the cables enclosed, is made up into a string as the heater housing 18 is lowered into the well. The cable cross-over 34 through which the cables 26 and 28 pass horn inside to the outside thereof is then connected to the upper end of the protective tubing 32 and this section of the string is pressurized through the valve 25 in the cross-over 34 to approximately 200 p.s.i.g. to insure against collapsing of this section during normal operations when the annulus pressure is about 500 p.s.i.g., or higher.

The perforated pup-joint 38 is next placed in the string to allow the well bore to be flushed from the bottom upward with a gas, for example, carbon dioxide, prior to pulling the heater from the borehole following the completion of the in situ combustion operation. Successive joints of well tubing 36 are then made up in the tubing string with the cables 26 and 28 banded or tied to the string by ties 21, preferably, at each joint, until the heating section 27 of the heater housing 13 is lowered to a total depth adjacent to the formation to be treated, as shown in FIG. l of the drawing.

The braden head 16 of the well 10 is installed with the cables 26 and 28 passing therethrough and packed olf pressure tight. The power cable 26 is connected to the autotransformer and the signal cable 23 is connected to the temperature indicating device and two of the conductors of the signal cable 23 also are connected to the balancing network 162 so as to couple the balancing network 162 to one of the three thermocouples 30, 31 and 33 in the heating section 27 through the three-position switch 164.

With the down hole equipment in place, an air compressor (not shown) is operated to force air through the pipe 42 into the annular space between the well casing 14 and the outside of the strings of tubing 32 and 36. The air may be injected into the well through the pipe 42 at a relatively high initial pressure to remove the borehole tiuid and then reduced by a substantial amount. The voltage is then applied to the power cable 26 to supply energy to the heater elements 140 and 142. The system of the present invention has been so designed that, if desired, full load may be applied to the heater elements 146 and 142 at any time without regard to a thermal gradient.

The temperature indicating device 160 continuously and simultaneously indicates the temperature of five longitudinally spaced points in the borehole, at the thermistor 76 which provides indications of the temperature in the cable cross-over 34, at the first thermocouple 29 which provides an indication of the temperature in the heater head 24 and at the thermocouples 3i), 31 and 33 which provide indications of the temperature at the upper, middle and lower portions, respectively, of the heating section 27. The temperature detected by one of the thermocouples 30, 31 or 33 may be recorded in the strip-chart recorder 167 by placing the movable arm m iu contact with one of the stationary contacts a, b or c, respectively, of the three-position switch 164. As shown in FIGURE 3 of the drawing, the movable arm m is in contact With iixed contact b of the three-position switch 164 so that a record will be provided of the temperature detected by the thermocouple 31 located in the middle portion of the heating section 27. Thus, the voltage produced by the thermocouple 31 will be applied to the balancing network 162. The potentiometer 166 has a constant voltage applied to the tWo fixed terminals 168 and 176 to produce a range oi' voltages which may be used to balance the voltage produced by the thermocouple 31. The required voltage to balance-the balancing network 162 is derived from the adjustable tap 172 of the potentiometer 166, the portion of which is adjusted by the first reversible motor 174. When the balancing network 162 is in equilibrium the iirst motor 174 is stationary, but when the voltage produced by the therrnocouple 31 increases, the unbalanced condition will cause the first motor 174 to move the tap 172 in one direction until a balancing voltage is reached, and when the voltage produced by the thermocouple 31 is decreased, the unbalanced condition will cause the first motor 174 to move the tap 172 of the potentiometer 166 in the opposite direction until a balancing voltage is reached, so as to again produce au equilibrium condition in the balancing network 162 at which time the rst reversible motor 174 ceases to .drive the adjustable tap 172. Since the position of the tap 172 of the potentiometer 166 is an indication of the temperature in the middle portion of the heating section 27, a marker mounted on the tap 172 of the potentiometer 166 produces a graph 176 on the strip chart 178 of the recorder 167.

v In order to control the temperature range within which the heater elements 14) and 142 are to operate the first and second electrical contacts 136 and 188 are positioned at spacedV apart points so as to cause an increase in the voltage applied to the conductors 17 and 19 of the power cable 26 when the temperature in the heating section 27 falls to the minimum desired temperature and to cause a decrease in the voltage applied to the conductors 17 and 19 of the power cable 26 when the temperature in the heating section reaches the desired maximum temperature. When the desired maximum temperature is produced in the heating section 27 the movable contact 184 mounted on the tap 172 is in electrical contact with the second electrical contact 183 which shorts out the coil 1% of the relay 192. With the coil 194 shorted the second movable arm 204 contacts the third xed contact 292 of the relay 192 to complete the circuit from the second llO volt source 193 through the lirst normally closed motor switch 26S to the second terminal 209 of the second reversible motor 266. The variable tap 156 which carries the actuatirig arm 214 of the autotransformer 155 is then driven to the right as illustrated in FIGURE 3 of the drawing to decrease the voltage between the first and second variable taps 156 and 158 and thus between conductors 17 and 19 of the power cable 26. The first variable tap 156 will continue to move toward the right until the actuating arm 214 opens the irst motor switch 26S thusl to provide the minimum voltage applied to the conductors 17 and 19 of the power cable 26. The decrease in voltage between the conductors 17 and 19 produces a decrease in the arnount of energy supplied to the heating section 27. This will tend to cause a decrease in the temperature in the heating section 27 and thus Vthe rst reversible motor 174 in response to the output from the balancing network 162 will drive the tap 172 of the potentiometer 166 toward the first electrical Contact 186. When the tap 172 reaches the point on the potentiometer 166 corresponding to the desired minimum temperature the movable Contact 184 will contact the first electrical contact 136 to energize the coil 194 of the relay 192. When the coil 194 is energized the rst movable arm 198 contacts the lirst fixed contact 196 which continues to connect the coil 194 to the first 110 volt source 191 even after the movable contact 184 hasy been disconnected from the first electrical Contact 186. When the coil 194 is energized the second movable arm 204 of the relay 192 is in contact with the second ixed Contact 200 of the relay 192. With the movable arm 204 contacting the second iixed Contact 260, the second ll() volt source 193 is connected through the second normally closed motor switch 212 to the third terminal 210 of the second reversible motor 2%. The variable taps 156 and 158 of the autotransformer 155 will now move toward the left as illustrated in FIGURE 3 of the drawing Lmtil the actuating arm 214 on the variable tap 156 opens the second motor switch 212 thus to provide the maximum voltage applied to the two conductors 17 Vand 19 of the power cable 26. This maximum voltage will be applied continuously to the heating elements and 142 tmtil the desired maximum temperature is reached in the heating section 27 at which time the movable contact 184 will contact the second electrical contact 188, as shown in FIG. 3 of the drawing to short circuit the coil 194 of the relay 192 to again place the second movable arm 2li-- in contact with the third Xed contact 202 of the relay 192 to cause motor 266 to drive the first and second variable taps 156 and 155 or the autotransformer toward the right in the direction which decreases the voltage applied to the conductors 17 and 19 of the power cable 26. This operation is repeated for any desired length of time.

In one well in which an in situ combustion operation was performed with the heater of the present invention, an initial pressure of 680 p.s.i.g. was maintained in the well for about one hour after which time the pressure was decreased to 500 p.s.i.g. at which level it was held constant for five hours. During this period of time, the fluid in the well bore was displaced into the subsurface formations. The air injection rate during this period was about 340,000 cubic feet per day. After about two days of operation, the air injection ratewas reduced and varied between about 160,000 to 200,000 cubic feet per day for the remainder of the in situ combustion operation. The electrical power to the heater was turned on and increased over a period of ten hours to a load of 360 volts and 28.2 amperes, with a recorded temperature of approximately 870 F. After the borehole temperature reached approximately 870 F., the temperature in the well bore increased without an increase of power input to the electrical heater. The power to the heater was then cut off and the temperature continued to increase to about l,350 F. and then gradually decreased over a period of about three hours to approximately 950 F. At this point the power was again turned on and gradually increased to 440 volts and 33.5 amperes where it remained for about l2() hours, i.e. until the in situ combustion operation was completed.

Although in the above-mentioned in situ combustlon operation of one well the heater operated at 950 F. for a period of 120 hours, it has been successfully operated at higher temperatures for longer periods of time and 1s capable of sustained operation at l,500 F.

Accordingly, it can be seen that an improved heater for the recovery of petroleum by thermal methodshas been provided. Furthermore, the invention has provided a heater which is easily positioned in a borehole by the use of standard oil field supplies ordinarily found at a Well site which eliminates the need for expensive armored mul ti-conductor cables.

Obviously, many modifications and variations of the invention as hereinabove set forth may be made Without departing from the spirit and scope thereof and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. An electrical borehole heater comprising in combination an elongated heater housing having a heating section below a heat baille section, an electrically resistive element located in said heating section, a support frame for said element disposed Within said housing, said frame extending longitudinally over the lengaA of both said heating and heat baille sections, said comprising elongated ilat plates disposed transversely along a longitudinal axis to form a cruciform cross-section, a plurality of openings in said frame located in said heating section and near the longitudinal exterior edges of said plates, a plurality of annular insulators for supporting said resistive element and adapted to be inserted in said openings, electric circuit rneans in said housing for supplying electrical energy to said resistive element, and means for suspending said housinJ at a predetermined location in a borehole.

2. An electrical borehole heater as set forth in claim 1 wherein said electrically resistive element is wound in the forni of a first helix of a irst constant radius which first helix is Wound in the form of a second helix of a second constant radius substantially greater than the iirst radius, and the internal diameter of each of said annular insulators is substantially equal to the external diameter of the first helix.

3. An electrical borehole heater as set forth in claim l wherein said sus ending means includes a string of protective tubing connected to the upper portion of said heater housing coaxial therewith, and wherein Said electric circuit means includes a power cable disposed within said protective tubing, said string of protective tubing including means for providing a fluid-tight seal between the interior and exterior thereof.

4. An electrical borehole heater as set forth in claim 3 further including means disposed within said protective tubing for supporting said power cable.

5. An electrical borehole heater as set forth in claim 4 wherein said supporting means includes a chain and a plurality of longitudinally spaced ties attaching said power cable to said chain.

6. An electrical borehole heater as set forth in claim 1 further including temperature responsive means in said heater housing, and means responsive to said temperature responsive means for controlling the flow of electrical energy to said resistive element.

References Cited in the tile of this patent UNITED STATES PATENTS 1,140,982 Huff May 25, 1915 1,841,332 Kranz Jan. 12, 1932 2,771,140 Barclay et al. Nov. 20, 1956 2,792,895 Carpenter May 2l, 1957 2,836,248 Covington May 27, 1958 FOREIGN PATENTS 316,463 Great Britain Aug. 1, 1929 331,436 Great Britain `luly 3, 1930 

1. AN ELECTRICAL BOREHOLE HEATER COMPRISING IN COMBINATION AN ELONGATED HEATER HOUSING HAVING A HEATING SECTION BELOW A HEAT BAFFLE SECTION, AN ELECTRICALLY RESISTIVE ELEMENT LOCATED IN SAID HEATING SECTION, A SUPPORT FRAME FOR SAID ELEMENT DISPOSED WITHIN SAID HOUSING, SAID FRAME EXTENDING LONGITUDINALLY OVER THE LENGTH OF BOTH SAID HEATING AND HEAT BAFFLE SECTIONS, SAID FRAME COMPRISING ELONGATED FLAT PLATES DISPOSED TRANSVERSELY ALONG A LONGITUDINAL AXIS TO FORM A CRUCIFORM CROSS-SECTION, A PLURALITY OF OPENINGS IN SAID FRAME LOCATED IN SAID HEATING SECTION AND NEAR THE LONGITUDINAL EXTERIOR EDGES 